3GPP LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station transmits a synchronization signal (Synchronization Channel: SCH) and broadcast signal (Broadcast Channel: BCH) using predetermined communication resources. A terminal secures synchronization with the base station by catching an SCH first. After that, the terminal acquires parameters specific to the base station (e.g. frequency bandwidth) by reading BCH information (see Non-Patent Literatures 1, 2 and 3).
Furthermore, after completing the acquisition of parameters specific to the base station, the terminal makes a connection request to the base station to thereby establish communication with the base station. The base station transmits control information to the terminal with which communication is established via a PDCCH (Physical Downlink Control CHannel) as required.
The terminal then makes a “blind decision” on each of a plurality of pieces of control information included in the received PDCCH signal. That is, the control information includes a CRC (Cyclic Redundancy Check) portion and this CRC portion is masked with a terminal ID of the transmission target terminal in the base station. Therefore, the terminal cannot decide whether or not the control information is directed to the terminal until the CRC portion of the received control information is demasked with the terminal ID of the terminal. When the demasking result shows that the CRC calculation is OK in the blind decision, the control information is decided to be directed to the terminal.
Furthermore, in 3GPP LTE, ARQ (Automatic Repeat Request) is applied to downlink data from a base station to a terminal. That is, the terminal feeds back a response signal indicating the error detection result of the downlink data to the base station. The terminal performs a CRC on the downlink data and feeds back ACK (Acknowledgment) when CRC=OK (no error) and NACK (Negative Acknowledgment) when CRC=NG (error present) as a response signal to the base station. A binary phase shift keying (BPSK) scheme is used for modulation of the response signal (that is, the ACK/NACK signal). Further, an uplink control channel such as a physical uplink control channel (PUCCH) is used for feedback of the response signal. When the received response signal represents NACK, the base station transmits retransmission data to the terminal.
Here, the control information (that is, downlink assignment control information) transmitted from the base station includes resource assignment information including resource information assigned from the base station to the terminal and the like. The aforementioned PDCCH is used for transmission of this control information. This PDCCH is made up of one or a plurality of L1/L2 CCHs (L1/L2 Control Channels). Each L1/L2 CCH is made up of one or a plurality of CCEs (Control Channel Elements). That is, a CCE is a base unit when control information is mapped to a PDCCH. Furthermore, when one L1/L2 CCH is made up of a plurality of CCEs, a plurality of continuous CCEs are assigned to the L1/L2 CCH. The base station assigns an L1/L2 CCH to the resource assignment target terminal according to the number of CCEs necessary to notify control information for the resource assignment target terminal. The base station then transmits control information mapped to physical resources corresponding to the CCEs of the L1/L2 CCH.
Here, each CCE has a one-to-one correspondence with a component resource of the PUCCH. Thus, the terminal that has received the L1/L2 CCH can implicitly specify component resources of the PUCCH corresponding to the CCEs configuring the L1/L2 CCH, and transmits the response signal to the base station using the specified resources. Accordingly, downlink communication resources can be efficiently used.
As shown in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread by a ZAC (Zero Auto-correlation) sequence having a Zero Auto-correlation characteristic, Walsh sequence and DFT (Discrete Fourier Transform) sequence on the time axis, and code-multiplexed within the PUCCH. In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4 and (F0, F1, F2) represents a DFT sequence having a sequence length of 3. As illustrated in FIG. 1, in the terminal, a response signal of ACK or NACK is primary-spread in a one single carrier frequency division multiple access (1 SC-FDMA) symbol on a frequency axis by a ZAC sequence (having a sequence length of 12). Next, the primary-spread response signal is subjected to inverse fast Fourier transform (IFFT) in association with W0 to W3 and F0 to F2. The response signal spread by the ZAC sequence having the sequence length of 12 on the frequency axis is transformed into a ZAC sequence having a sequence length of 12 on the time axis through the IFFT. The signal which has been subjected to the IFFT is secondary-spread using Walsh sequences (having a sequence length of 4) and DFT sequences (having a sequence length of 3).
Here, response signals transmitted from different terminals are spread using sequences corresponding to different cyclic shift indices or different orthogonal cover (OC) indices (that is, a set of a Walsh sequence and a DFT sequence). Therefore, the base station can demultiplex a plurality of code-multiplexed response signals using a conventional despreading process and a conventional correlation process (see Non-Patent Literature 4).
Further, the standardization of 3GPP LTE-advanced that realizes faster communication than 3GPP LTE has started. A 3GPP LTE-advanced system (which may also be hereinafter referred to as “LTE-A system”) follows the 3GPP LTE system (which may also be hereinafter referred to as “LTE system”). In order to realize a downlink transmission rate of a maximum of 1 Gbps or above, 3GPP LTE-advanced is expected to introduce base stations and terminals capable of communicating at a wideband frequency of 40 MHz or above.
In an LTE-A system, to simultaneously realize communication at an ultra-high transmission rate several times as fast as a transmission rate in an LTE system and backward compatibility with the LTE system, a band for the LTE-A system is divided into “unit bands” of 20 MHz or less, which is a supported bandwidth for the LTE system. That is, the “unit band” is a band having a width of maximum 20 MHz and defined as a base unit of a communication band. Furthermore, a “unit band” in a downlink (hereinafter, referred to as “downlink unit band”) may be defined as a band divided by downlink frequency band information included in the BCH broadcasted from the base station, or a band by a spreading width when the downlink control channel (PDCCH) is spread and arranged in the frequency domain. Further, a “unit band” in an uplink (hereinafter, referred to as “uplink unit band”) may be defined as a band divided by uplink frequency band information included in the BCH broadcasted from the base station, or as a base unit of a communication band of 20 MHz or less, which includes a physical uplink shared channel (PUCCH) region near the center thereof and PUCCHs for the LTE at both ends thereof. Furthermore, in 3GPP LTE-Advanced, the “unit band” may also be expressed as “component carrier(s)” in English.
Meanwhile, the uplink control channel (PUCCH) is also used for transmission of a scheduling request (SR) (which may be represented by a scheduling request indicator (SRI)) which is an uplink control signal indicating that uplink data to be transmitted from the terminal side has been generated. When a connection with the terminal has been established, the base station individually assigns a resource to be used for transmission of the SR (hereinafter, referred to as “SR resource) to each terminal. Further, an on-off keying (OOK) scheme is applied to the SR, and the base station detects the SR from the terminal based on whether or not the terminal is transmitting an arbitrary signal using the SR resource. Further, the SR is spread using the ZAC sequence, the Walsh sequence, and the DFT sequence, in the same manner as the above mentioned response signal.
In the LTE system, the SR and the response signal may be generated in the same sub frame. In this case, when the terminal code-multiplexes and transmits the SR and the response signal, a peak to average power ratio (PAPR) of a synthesized waveform of a signal transmitted from the terminal significantly deteriorates. However, in the LTE system, since importance is put on amplification efficiency of the terminal, when the SR and the response signal have been generated in the same sub frame at the terminal side, the terminal transmits the response signal using the SR resource previously individually assigned to each terminal, without using a resource (hereinafter, referred to as “ACK/NACK resource”) used for transmission of the response signal. Thus, the PAPR of the synthesized waveform of the signal transmitted from the terminal can be reduced. At this time, the base station detects the SR from the terminal based on whether or not the SR resource is being used. In addition, the base station determines whether or not the terminal has transmitted either ACK or NACK, based on a phase (that is, a BPSK demodulation result) of a signal transmitted through the SR resource (the ACK/NACK resource when the SR resource is not used).
Further, in the LTE-A system, the terminal is assumed to include a plurality of transmitting antennas, and it is being discussed to apply space code transmit diversity (SCTD) (which is also called spatial orthogonal-resource transmit diversity (SORTD)) using a plurality of different code resources for the SR or the response signal. In the SCTD, for example, the base station assigns two ACK/NACK resources to one response signal, and the terminal transmits the same response signal assigned to each of different code resources through two antennas (see Non-Patent Literature 5).